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| Filtering PWM to smooth DC |
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| wraper:
--- Quote from: Rapsey on September 22, 2018, 08:12:48 pm ---Kinda surprising to see that, I guess 100% PWM isn't the same as continuous 12V DC after all. --- End quote --- Even when you directly connect the fan through that resistor, you still will have a voltage drop over it. |
| Rapsey:
--- Quote from: wraper on September 22, 2018, 08:29:14 pm --- --- Quote from: Rapsey on September 22, 2018, 08:12:48 pm ---Kinda surprising to see that, I guess 100% PWM isn't the same as continuous 12V DC after all. --- End quote --- Even when you directly connect the fan through that resistor, you still will have a voltage drop over it. --- End quote --- I wasn't talking about the voltage drop, rather the ripple in the output. Then again I don't know how much of that is from the PWM + RC LPF and how much is from the power supply itself. |
| MiDi:
If the PWM frequency is around 500Hz and as the screens show ~300Hz ripple, I would suggest that this ripple is not from PWM (100%) and is the commutating frequency of the fan. As they are quite near together this could be the cause for the noise, would suggest it was at around 200Hz and/or 800Hz... |
| Rapsey:
--- Quote from: MiDi on September 22, 2018, 08:42:54 pm ---If the PWM frequency is around 500Hz and as the screens show ~300Hz ripple, I would suggest that this ripple is not from PWM (100%) and is the commutating frequency of the fan. As they are quite near together this could be the cause for the noise, would suggest it was at around 200Hz and/or 800Hz... --- End quote --- The rated speed for the fan I used there (a MF50151V1 this time) is 5000 rpm (or ~83 Hz), though at 11V it would be a little less than that. I'll continue to experiment with different resistors/capacitors/fans, probably try an LC or RLC filter too. EDIT: Looks like you were right. Adding friction to slow the fan down widens the period of the entire wave form (both the sine and the spike) so they certainly don't have anything to do with the PWM frequency. |
| Zero999:
More than one RC stage can be used for a smoother output. A smaller value of R could be used to reduce the voltage drop, but make it too small, compared to the load and the PWM will no longer work. The capacitor will simply charge up with huge current spikes and the output will sit near to the full supply voltage, even with a fairly low duty cycle. Attached is a simulation which takes things to the extreme R = 1R, C = 1000µF and a load resistance of 80R to be close to the 170mA. The PWM frequency is a bit higher at 2kHz and a duty cycle of 20%. The voltage across the load settles at 13V, with 830mA current spikes being drawn each pulse. Rsw is a voltage controlled resistor which goes open circuit, well not quite but 1GΩ, when V1 drops below 13.8V and nearly closed circuit i.e. 1µΩ, when V1 is 13.8V. It's there to mimic the switching action of the PWM transistor. Without it, V1 will look like a low impedance and discharge back into it via R1, when the voltage drops. I could have used a voltage controlled switch or MOSFET but this is quicker to simulate and it's good to teach people about this. I could have also combined R1 with Rsw by changing the statement to R = if(V(PWM)<13.8,1G,1), but having a separate resistor makes the schematic easier to read. |
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